
SCIENCE132
a low-pitched sound. On the other
hand, a whistle has a high frequency
and therefore, produces a sound of
higher pitch (Fig. 10.18). A bird makes
a high-pitched sound whereas a lion
makes a low-pitched roar. However, the
roar of a lion is very loud while the
sound of the bird is quite feeble.
Every day you hear the voices of
children and adults. Do you find any
difference in their voices? Can you say
that the frequency of the voice of a child
is higher than that of an adult?
Usually the voice of a woman has
a higher frequency than that of a
man.
10.6 Audible and Inaudible
Sounds
We know that we need a vibrating body
for the production of sound. Can we
hear the sound of all vibrating bodies?
The fact is that sounds of frequencies
less than about 20 vibrations per second
(20 Hz) cannot be detected by the human
ear. Such sounds are called inaudible.
On the higher side, sounds of frequencies
higher than about 20,000 vibrations per
second (20 kHz) are also not audible to
the human ear. Thus, for human ear
,
the range of audible frequencies is
roughly from 20 to 20,000 Hz.
Some animals can hear sounds of
frequencies higher than 20,000 Hz.
Dogs have this ability. The police use
high frequency whistles which dogs can
hear but humans cannot.
The ultrasound equipment, familiar
to us for investigating and tracking
many medical problems, works at
frequencies higher than 20,000 Hz.
10.7 Noise and Music
We hear different types of sounds
around us. Is the sound always
pleasing? Does a sound sometimes
cause discomfort to you? Some sounds
are pleasant to the ear, whereas some
are not.
Suppose construction work is going
on in your neighbourhood. Are the
sounds coming from the construction
site pleasing? Do you enjoy the sounds
produced by horns of buses and trucks?
Such unpleasant sounds are called
noise. In a classroom, if all the students
speak together, what would the sound
produced be called?
On the other hand you enjoy sounds
from musical instruments. Musical
sound is one which is pleasing to the
ear. Sound produced by a harmonium
is a musical sound. The string of a sitar
also gives out a musical sound. But, if
a musical sound becomes too loud,
would it remain melodious?
10.8 Noise Pollution
You already know about air pollution.
Presence of unwanted gases and
particles in air is called air pollution.
Similarly, presence of excessive or
unwanted sounds in the environment
is called noise pollution. Can you list
some sources of noise pollution? Major
causes of noise pollution are sounds of
vehicles, explosions including bursting
of crackers, machines, loudspeakers etc.
What sources in the home may lead to
noise? Television and transistor radio
at high volumes, some kitchen
appliances, desert coolers, air
conditioners, all contribute to noise
pollution.